1. Field of the Invention
The present invention relates to a lens barrel, and more particularly, to a lens barrel equipped with a mobile-object detection apparatus which detects relative movement between two members such as a fixed frame and rotary frame.
2. Description of the Related Art
Conventionally, a lens barrel or the like which includes multiple frame members incorporates a mobile-object detection apparatus which detects relative movement between two members.
Frame members which make up a typical lens barrel includes, for example, a fixed frame fastened to a fixed part or the like of a camera body, a rotary frame housed in the fixed frame in such a way as to be able to rotate relative to the fixed frame and move relative to the fixed frame along an optical axis, plural lens frames housed in the rotary frame in such a way as to be able to move along the optical axis, but with rotation relative to the rotary frame restricted.
The plural lens frames hold lens groups each of which contains one or more lenses. During a zoom operation, as the rotary frame is rotated in the fixed frame, the lens frames in the rotary frame advance or retreat along the optical axis along with the rotation of the rotary frame, thereby placed at predetermined zoom positions on the optical axis.
During an autofocusing operation, out of the lens frames in the rotary frame, a focusing lens frame which holds a focusing lens group advances and retreats along the optical axis, thereby performing a focusing operation.
Some conventional lens barrels configured as described above are equipped with a mobile-object detection apparatus which detects relative movement between two members, such as a fixed frame and rotary frame, which move relative to each other. Such a mobile-object detection apparatus is generally equipped with a photodetector such as a photoreflector (photosensor).
The conventional lens barrel is designed such that the rotary frame advances and retreats along the optical axis, rotating in the fixed frame. By detecting an amount of rotation of the rotary frame, it is possible to detect an amount of movement of the rotary frame relative to the fixed frame.
Thus, the mobile-object detection apparatus of the conventional lens barrel detects an amount of relative movement of the rotary frame using a photoreflector (photodetector).
That is, the conventional lens barrel includes a motor (zoom motor) which advances and retreats the rotary frame to/from a predetermined zoom position and a reduction gear train which, being linked to a gear fixed to an output shaft of the motor, reduces rotary driving of the motor.
Thus, some conventional lens barrels are designed to detect the amount of relative movement of the rotary frame by detecting RPM of the motor using a photoreflector (photodetector).
Normally, a lubricant such as grease is applied to the reduction gear train to reduce friction, to muffle sound, and so on.
However, the photoreflector (photodetector) tends to be susceptible to lubricants such as grease.
For example, grease may fly off the reduction gear train when the motor rotates. In such a case, if the grease flying off when the motor rotates attaches to the photoreflector or an impeller placed between a light-emitting unit and light-receiving unit of the photoreflector (photodetector), it may become difficult to detect the RPM of the motor, and thus the amount of relative movement of the rotary frame. If the amount of relative movement of the rotary frame cannot be detected, the zoom position cannot be found accurately as designed.
To deal with this, a reduction gear unit disclosed in Japanese Patent Application Laid-Open Publication No. 2004-191434 is configured such that an impeller and reduction gear train are housed in compartments isolated from each other to prevent grease from flying off the reduction gear train to the impeller or a photoreflector.
Regarding means to detect the amount of relative movement of the rotary frame, various means are available in addition to the above-described means which involves detecting the RPM of the motor (zoom motor). For example, it is conceivable to place a photoreflector (photodetector) in an opening provided in the rotary frame, emit light from a light-emitting unit of the photoreflector to the rotary frame, and receive light reflected from the rotary frame using a light-receiving unit of the photoreflector. Such a configuration can also detect the amount of relative movement of the rotary frame.